1. Field of the Invention
The present invention relates to a device for the regulation of the flow of ATM cells within an ATM (asynchronous transfer mode) type of packet switch that can be used to resolve the difficulty of making point-to-multipoint circuits for sporadic flows in ATM.
2. Discussion of Background
The ATM switching technique is internationally recognized as one that will become the basis of wideband telecommunications networks in the near future, in local area networks or LANS, metropolitan area networks or MANS and wide area networks or WANs as well as in networks for distribution to users (both professional and residential).
This technique consists of the packet switching over of the virtual circuits conveying protocol and user information elements. The ATM technique, which is based on a autonomously identified, fixed-size quantum of information known as a cell, enables the making of machines with a variety of roles and characteristics and with high performance values.
An ATM cell is formed by two main parts:
a header identifying the virtual circuit to which the cell belongs, protected by an error detector/corrector; the size of a header is fixed at 5 bytes by the ITU (International Telecommunications Union);
the payload, conveying the information proper to the user and to the matching of this information with the ATM; the size of the payload is fixed at 48 bytes by the ITU.
The types of traffic conveyed by ATM networks can be classified under four main categories:
a first category of traffic known as CBR traffic or constant bit rate traffic corresponds to the traffic associated with a constant bit rate source which therefore sends out cells (corresponding to sound, images encoded at constant bit rate etc.) at regular intervals;
a second category corresponds to what is known as VBR traffic or variable bit rate traffic and corresponds to traffic associated with a source with a variable bit rate, the variation of which depends on the instantaneous characteristics of the source (this traffic consists of images encoded at variable bit rate, etc.);
a third category corresponds to a type of traffic known as UBR traffic or unspecified bit rate traffic and corresponds to traffic associated with a source whose characteristics are not specified on an a priori basis. This type of traffic is typically associated with sources of the type pertaining to the transfer of computer files. This type of traffic is highly sporadic, without major constraints in terms of transfer time but requires a very low rate of information loss. It is generally desired to use the entire passband left available by the CBR/VBR flows; this is what is called the "best effort" or BE operation; and
a fourth category corresponds to a traffic known as available bit rate or ABR traffic corresponding to the UBR traffic with, however, the sources being under the constraint of a control of flow proper to the ATM layer.
Typically, to the CBR and VBR type traffic there correspond statistical assumptions leading to relatively simple packet switching structures. These structures are characterized firstly by the absence of control over the flow and, secondly, by a small number of buffers or buffer memories, for a loss rate that is itself very small. To ensure that these statistical assumptions prevail, the traffic is "reserved" by a function for the allocation of the resources of the selector switch and is, if necessary, verified by a function access procedure with respect to the selector switch.
A type of architecture known as "output buffering" consists in placing buffer memories downline with respect to the multiplexing system (bus, ring, matrix, etc.). This architecture lends itself well to the making of CBR and VBR point-to-multipoint circuits since it is enough to simultaneously select several outputs to make a multiple copy of only one cell. The point-to-multipoint circuits correspond to circuits with one input port and several output ports, each output port delivering the same information. They are used to provide services requiring the general or selective broadcasting of information to several addressees, for example the broadcasting of image and sound, local network type of routing protocols, etc.
The UBR traffic differs from the CBR and VBR types of traffic in that the bit rate of the sources is not under constraint or under little constraint. It is then no longer possible to rely on stable short-term statistical behavior. The presence of a UBR/ABR flow requires "large" buffers owing to the sporadic nature of the traffic. These large buffers have a memory capacity typically equal to some thousands to some tens of thousands of cells. Depending on the architecture of the packet switching, it is necessary to add output buffers thereto and it may be desired to keep these output buffers "small" for reasons of speed, hence of cost of the memory. These small buffers have a memory capacity typically of some tens to some hundreds of cells. In this case, an internal flow control device enables the blocking of the output of the "large" buffers when the "small" buffers are full. Then, end-to-end transportation protocols are relied upon to limit the load offered to the limits of capacity of the "large" buffers.
The location of these "large" buffers differs according to the architecture. They are generally located at the input (input buffering) or at the center (central buffering) of the packet switch.
The ABR traffic introduces a flow control between the sources and the selector switches (or the destinations) enabling constraints to be placed on the sources within the limits that can be accepted by the installations of the network and thereby ensure overall loss-free operation.
The physical implementation of the point-to-multipoint circuits within ATM packet switches/selector switches is characterized by its level of complexity and hence its cost, and by its performance level as regards the bit rate offered, the repeated use of packet switching resources, multiple copies of the cells, etc. The present modes of implementation of such circuits propose physical solutions that are complex and therefore costly.